For example, in an image forming apparatus, such as an electrophotographic copying machine, a printer, and a facsimile machine, an image signal is converted into an electric signal in accordance with a density of image information of the original document, based on which an electrostatic latent image is formed on a photosensitive body using a laser beam or the like. Then, the electrostatic latent image is developed into a developer image and transferred onto a sheet, after which the developer image is fused with heat generated by a heater of a thermal fuser to be fixed thereon. In this type of fuser that fuses an image onto the sheet with heating, the heater (hereinafter, referred to as fusing heater) of the thermal fuser is provided as a load. In the fusing heater, a heater lamp, such as a halogen lamp, and a heating resistor or the like is used as a heat source. The fusing heater is enclosed in a pair of fusing rollers for nipping and transporting a sheet on which the developer image is to be fused. More specifically, one or more than one fusing heater each having power ranging from some hundreds Watts to approximately two thousands Watts is provided inside either or both of the pair of fusing rollers. In case of a high-speed image forming apparatus, a fusing heater having a larger capacity is used. Further, the pair of fusing rollers are kept at a predetermined temperature by controlling power supplied to the fusing heater using a fusing heater's ON/OFF signal which is generated based on a detection result of a temperature sensor provided in such a manner to touch the surface of the pair of fusing rollers.
When the image forming apparatus has a large load having positive characteristics with respect to temperatures as a subject component to which power is supplied under control, a large current (hereinafter, referred to as rush current) passes through the load immediately after the power supply begins. In the following, how the rush current flows into the load and how the power source voltage drops as the rush current passes through the load will be explained using a halogen heater of the fuser as an example and with reference to FIG. 7.
As indicated by a curve (a), when a heater signal assumes an ON state, power is supplied to the halogen heater from a commercial power source. Since a resistance value of the halogen heater has positive characteristics with respect to temperatures, in other words, it becomes larger as the temperature of the halogen heater rises, if a current has not been supplied to the halogen heater, the halogen heater has quite a small resistance value. Generally, a resistance value at such a low temperature is 1/10 of the resistance value when heated. Since power is supplied to the halogen heater having such a small resistance value, a rush current I.sub.1 (a peak value of a half cycle of the current in the initial stage) flows into the halogen heater immediately after the power supply starts as indicted by a curve (c).
The halogen heater is heated as the current flows in and a temperature of the same rises and so does the resistance value. As the resistance value rises, the current flowing into the halogen heater drops and converges to a normal current I.sub.0, and the halogen heater resumes to a normal state. A ratio of the rush current I.sub.1 to the normal current I.sub.0, I.sub.1 /I.sub.0, ranges from several to ten times. In case of FIG. 7, since the halogen heater is controlled to start to light on substantially at a zero crossing point of the power source voltage waveform, the rush current can be suppressed to a relatively small value.
On the other hand, as indicated by a curve (b) in the drawing, the rush current flowing into the halogen heater in the above manner causes a voltage drop .DELTA.V.sub.1 around an outlet of the commercial power source that supplies power to the image forming apparatus or in the other internal lines because of their own impedance. The curve (b) in the drawing represents an envelope of the voltage waveform when the voltage drops. After the current passing through the halogen heater has converged to the normal current, the voltage drop also converges to a small value .DELTA.V.sub.2. When the power supply to the halogen heater is cut, the voltage recovers an original voltage level V.sub.0.
Particularly, since the above rush current causes a significant voltage drop instantaneously, peripheral equipment or lighting equipment may be affected adversely. For example, when a voltage supplied to the lighting equipment drops, there possibly occurs a luminance flicker phenomenon (flicker). Recently, to suppress the occurrence of this phenomenon, apparatuses that consume large power with respect to the power source are regulated by the flicker test. The flicker test checks that a voltage at the power source end does not drop below a predetermined level because of the load provided in the apparatuses. In case of the image forming apparatus, the flicker test is carried out in two modes: a copy mode (the flicker test in this mode is referred to as short flicker test) and a standby mode (the flicker test in this mode is referred to as long flicker test). Thus, the flicker test is carried out based on regulation values set separately for each mode.
To suppress the problematic voltage drop, as is disclosed in Japanese Laid-open Patent Application No. 242644/1994 (Tokukaihei No. 6-242644), a control method, referred to as the phase control, for supplying power by increasing a conduction angle at which a current passes through the load step by step is known. However, when power is supplied to the load like the above-mentioned halogen heater through the phase control, the power supply starts at a point other than the zero crossing point of the voltage waveform, and a large voltage is applied abruptly to the load. Accordingly, not only the current waveform is distorted, but also conduction noise is emitted over a broad frequency band. The distortion of the current waveform adversely affects the surroundings of an outlet of the commercial power source connected to an apparatus that supplies power to the internal load through the phase control, or other apparatuses connected to the other internal lines. Also, the emitted conduction noise causes a problem that it triggers a malfunction of peripheral electronic equipment.
To eliminate the above problems, regulation is imposed by a test referred to as the harmonics test. The harmonics test checks how bad the distortion of the current waveform of FIG. 8 is with respect to the original waveform. In practice, whether a coefficient in each order of the harmonics obtained by subjecting the current waveform to the Fourier analysis is within predetermined regulation values or not is tested, and the second through fortieth harmonics are checked. The safety regulation requires the image forming apparatus to maintain the harmonic noises within the predetermined regulation values in a copy mode where a normal image is formed.
Various countermeasures are proposed to clear these regulations. For example, aforementioned Japanese Laid-open Patent Application No. 242644/1994 (Tokukaihei No. 6-242644) also discloses a technique for suppressing the occurrence of the rush current by increasing the conduction angle step by step using a softstart circuit employing a bi-directional thyristor (also known as TRIAC). When this technique is adopted, the voltage drop is suppressed effectively, but since the conventional phase control is effected, the current waveform is distorted considerably, thereby causing a large amount of conduction noise. To eliminate the adverse effect of the conduction noise on the other apparatuses, an expensive noise filter must be provided to the power source line. Thus, there is a problem that the cost is undesirably increased. Moreover, the problem of the distortion of the current waveform remains unsolved.
In the following, how the time length of the phase control period generally affects the aforementioned voltage drop, distortion of the current waveform, and conduction noise will be explained with reference to Table 1 below and FIG. 9.
TABLE 1 ______________________________________ HARMONICS PHASE FLICKER (DISTORTION CONTROL (VOLTAGE OF CURRENT CONDUCTION PERIOD Tph DROP) WAVEFORM) NOISE ______________________________________ LONG DOWN UP UP SHORT UP DOWN DOWN ______________________________________
Here, the phase control is effected in such a manner that the power supply to the load is started after a predetermined period (delay time) from the zero crossing point of the voltage waveform and stopped on the zero crossing point during a predetermined period Tph (FIG. 9) after the current is allowed to flow. Subsequently, the phase control is switched to the zero crossing control for fully passing the current continuously.
In this case, as is understood from Table 1, the longer the phase control period, the smaller the voltage drop, and the shorter the phase control period, the larger the voltage drop. On the contrary, the longer the phase control period, the higher the level of the distortion of the voltage waveform and the conduction noise, and the shorter the phase control period, the lower the level of the distortion of the current waveform and the conduction noise.